Strange to say this, but at two hours and twenty-two minutes, “Harry Potter and the Prisoner of Azkaban” is too short. Director Alfonso Cuarón, taking over for Chris Columbus, helmer of the first two “Potter” adventures, expressed concern that the films were getting too unwieldy in their efforts to remain faithful to J.K. Rowling’s novels, and so, along with writer Steve Kloves (who’s scripted all of these pictures), he’s tightened up the adaptation, creating a brisker, faster movie.

This new line of thought may please those who felt the first two “Potters” were too languid in their pacing, too lax, too uninterested in brevity. Me, however... Yes, I admired Cuarón’s decision - after all, “Sorcerer’s Stone” took a little too long to get rolling, and this new style fits the growing franchise, one with increasingly darker themes and less passion for whimsy. But the cost of a brisker story is a plot that feels disinterested in itself in certain key parts. There’s a major scene around two-thirds into the movie where the entire plot gets reworked, and frankly, it whips by a little too quickly, as if it’s not as vital as it truly is; when it’s over, we’re left wondering if we’re missing a few minutes of spare exposition.

That, however, is my lone complaint, for “Azkaban” still captures everything that’s right about this series. The “Potterverse” is still a wonderful place to visit, and this latest film still gives us plenty to enjoy. There’s all that eye candy (the set design is still quite impressive, and the visual effects are the best yet in the series), and more importantly, the story feels less like a standalone sequel than an important next chapter in an ever-evolving epic. These characters are growing, and the series is growing right along with them.

The film hints at changes to come right at the start, with Harry accidentally (and comically) unleashing some magic back in our world - a sign of wizard puberty, perhaps? It’s a bit of whimsy before the darkness; more reminders that our heroes are kids no more soon come with the rather frightening first appearance of the Dementors, soul-sucking phantoms hunting for a prison escapee. These creatures are one of many that suggest this third tale is more suited for Halloween. It’s darn near being an all-out horror movie.

The escapee mentioned above is Sirius Black (Gary Oldman), a wizard who’s connected somehow with the death of Harry’s parents and who may very well be heading to Hogwart’s to finish off the family. Which is not the sort of thing that’ll make your first year of teenagerhood any easier.

The story’s not all phantoms and murderers and other bump-in-the-night beings. There’s a delightful, gentle subplot involving the school’s newest professor, Lupin (David Thewlis), who was a close friend of Harry’s father and who becomes a welcome mentor for the young hero. There’s great interplay between Thewlis and Daniel Radcliffe, who’s gone from a great child actor to an even better teen actor; their scenes together crackle, making these quiet, personal moments the best parts of the film.

There’s also the usual great performances from the rest of the cast, all of whom showing that they’re fully comfortable in familiar roles yet never settling to sleepwalk through anything. The returning cast all perform on the high level we’ve come to expect from this exceptional cast (Rupert Grint remains my favorite young actor, his Ron my favorite character in the series), and the newcomers (which also include Michael Gambon, replacing Richard Harris as Dumbledore, and Emma Thompson, as a hilariously batty psychic) again prove up to the challenge. The Potter series stands only next to “Lord of the Rings” as the best-acted, best-casted film series of the past few years.

That said, “Azkaban” is Cuarón’s film all the way. The director, who’s handled everything from the childhood fantasy “A Little Princess” to the adults-only Mexican drama “Y Tu Mamá También,” relishes the chance to take the Potter franchise in darker new directions. He wisely treats later sequences (involving a variety of monsters and such) as pure horror (albeit horror with a PG rating), and he even manages to add the right pinch of menace into otherwise harmless scenes. Watch as Thompson’s character goes from dotty goofball to a medium for evil and back again - it’s a brilliant mix of terror and comedy.

The result of Cuarón’s determination to stretch the series is a rich movie experience worthy of the Potter name. This is a fantastic yarn and a successful attempt to allow the characters and story to grow, not merely continue. Yes, there are a few bumps in the script, but it never affects the overall sense of magic (no pun intended). While not the best in the series, “Azkaban” is still a great adventure, a work that’s marvelous as part of the franchise and on its own as well. Here’s to waiting for Part Four.